• What level of SOx emissions from the FCC should we expect from current catalyst additives?



  • Rick Fisher, Johnson Matthey, rick.fisher@matthey.com

    The answer to this question depends on several factors. Is the FCC full burn or partial burn? Does the FCC run in gasoline mode or diesel mode?

    A SOx additive has three basic components.
    1. The sorbate: this is what captures the SO3, which is a mixed Mg/Al oxide.
    2. The oxidation package, normally a Cerium oxide which converts SO2 to SO3 so that it can be captured by the Mg/Al oxide (the Mg/Al oxide will only capture SO3).
    3. The S-release package: various transition metals to facilitate the reduction of the MSO4 back to MO, converting the SOx to H2S.

    The sorbate and oxidation reactions both occur in the regenerator and are thus dependent on the regenerator operation and conditions. The S-release reactions occur in the riser/reactor and are thus dependent on the riser/reactor conditions.

    SOx reduction in full-burn regenerators is generally very effective. It is favoured by the higher partial pressure of SOx and O2, higher catalyst circulation rates, higher catalyst replacement rates (younger inventory), lower regenerator temperatures, good air/catalyst mixing in the regenerator, and higher riser/reactor temperatures. Johnson Matthey’s Super SOxGetter II typically achieves SOx removal rates in excess of 80% for most full-burn units, and over 95% SOx removal has been maintainable in numerous full-burn units, and complete SOx elimination has been achieved in some full-burn units.

    SOx reduction in partial-burn regenerators is limited by the availability of oxygen for the oxidation reactions, converting reduced sulphur species (COS, H2S) to SO2 and then to SO3. The higher the CO content of the regenerator flue gas, the less effective a SOx additive will be. A rule of thumb to estimate the maximum SOx reduction achievable in a partial-burn regenerator is:

    Maximum achievable SOx reduction in partial burn = 100% - %CO*10
    For a partial-burn regenerator with a flue gas CO content of 6%, the maximum achievable SOx reduction would be approximately 40%. If higher SOx reduction is desired/required, the unit operation will most likely need to be moved to shallower partial burn (lower CO). LO-SOx PB XL is Johnson Matthey’s SOx additive specifically developed for use in FCCs operating in mid-deep partial burn. LO-SOx PB XL can substantially reduce the amount of additive required to attain a desired SOx target compared with standard SOx additive technologies.

    Units with two-stage regeneration (units with both a full-burn and partial-burn regenerator) will experience both scenarios described above, and the SOx reduction limit is almost always set by the SOx reduction that can be achieved in the partial-burn regenerator. Due to the limits of the partial-burn regenerator, LO-SOx PB XL is normally used in these applications.

    A few units experience issues with the S-release in the riser/reactor. When this occurs, it is almost always due to operating at lower riser/reactor temperatures (such as diesel mode). This is most usually seen at temperatures below 950°F (510°C). Johnson Matthey has developed super SOxGetter II DM to alleviate these issues and achieve comparable efficiency to Super SOxGetter II at higher riser temperatures.

    As you can see, many factors will ultimately determine the amount of SOx emissions reduction that can be achieved with the SOx additives available in the market today.



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